Customary eccentric mechanisms, crank mechanisms or cam mechanisms are used to translate a motion of rotation into a translational reciprocating motion or vice versa. Such mechanisms usually have a fixed stroke that corresponds to the eccentricity of the eccentric of the eccentric shaft/secondary shaft or the dimension of the crank of the crankshaft.
There also are crank mechanisms with adjustable strokes (HuTTE Taschenbuch fur Betriebsingenieure, Band II Fertigungsmaschinen, Verlag von Wilhelm Ernst & Sohn, 6.sup.th edition 1964, page 505). An adjustment bush with an eccentrically located bore is articulated on the eccentric, cylinder shaped crank pin of a so-called outside crankshaft and with the adjustment bush in turn being articulated on the connecting rod bearing of a connecting rod of a connecting rod press. By turning the adjustment bush on the crank pin, which can only be done when the press is at a standstill, the connecting rod stroke can be changed from a maximum size to a minimum size or vice versa.
However, there is a need especially for machines that work with an eccentric mechanism and a connecting rod stroke in which the stroke is adjustable during operation and in particular in a manner that ensures that the connecting rod stroke position after turning off the eccentric mechanism is identical to the one at the time of turning on the eccentric mechanism. A concrete example is the process of rotary friction welding of thermoplastic and elastic plastic materials. In these cases long profile belts with round or angular profiles, for example pentagonal profile sections, V-belt profile sections etc., made of such plastic material must be adjusted to finished size for the purpose of manufacturing such conveyor belts and then the ends of the profile belts must be welded by keeping one profile belt end in place while the other end of the belt is pressed against the stationary end of the belt and simultaneously is melted onto it via a translational reciprocating motion accompanied by the development of frictional heat. The same is true for the repair of such profile belts when they are torn. It is important that the ends of the belts are exactly aligned during the final welding process of the ends of the profile belts. Customary eccentric mechanisms whose connecting rods would provide the translational reciprocating motion of the end of one of the profile belts for the purpose of rotary friction welding cannot guarantee this because when such an eccentric mechanism or crank mechanism runs out, it arrives at its fixed eccentricity when at a standstill, which would not be desirable for the use of a friction welding device, for example.